1. Field of the Invention
The present invention relates to locating a deterioration and a fault of a transmission line in a system which transmits a signal, particularly an optical signal bidirectionally between a single device and a group of plural devices.
To develop broadband multimedia services including the Internet, CATV (Cable Television system) and VOD (Video On Demand), it is in great demand to construct an economical, high-speed and broadband access network system. As seen in the FTTH (Fiber To The Home), optical fibers are being utilized for subscriber lines as means to offer large-capacity communication services in order to achieve more high-speed data access and interactive information transmission from terminals at home. However, it is a problem in economical aspect to introduce optical fibers with large transmission capacity into general homes, compared with conventional metallic lines.
Therefore, the PDS (Passive Double Star) system is drawing attention in which an optical fiber extending from a station-side device is branched halfway into plural fibers, each connected with a subscriber device (may be simply called a subscriber) so as to offer bidirectional communication services economically between the station-side device and the subscribers. To perform bidirectional communication in the PDS system, it is necessary to multiplex a downward (station-side device to subscribers) signal with an upward (subscribers to station-side device) signal and to multiplex plural subscriber signals with each other.
The former bidirectional communication is realized by the time division multiplex (TDM) method or the wavelength division multiplex (WDM) method. As for the latter communication, the downward-direction multiplexing is realized by the TDM method in which a subscriber receives all the multiplexed signals transmitted from the station-side device and reads only the information within the time slot assigned to the subscriber. The upward-direction multiplexing is realized by the TDMA method in which the subscribers sharing the station-side device each control the signal transmission timing so that burst signals transmitted do not collide with each other.
In the PDS system which performs bidirectional communications with the transmission line branched halfway into plural lines between the station-side device and the subscribers, an apparatus and a method are in great demand to identify a faulty transmission line and determine the distance of a fault point of the transmission line.
2. Description of the Related Art
Since the refractive index of light changes greatly due to a fault of the transmission line such as a poor connection of connectors, disconnected optical transmission line, a light emitted into an end of a transmission line undergoes a xe2x80x9cFresnel reflectionxe2x80x9d at the fault point and returns back as a reflected light. Based on this phenomenon, a fault of a transmission line can be located by measuring the time since a test light is emitted into the transmission line until the light returns from the fault point as a reflected light (hereinafter, the time is called propagation time or delay time) and also the power of the reflected light.
FIG. 1 shows a configuration of a conventional supervising system. It shows an optical access system in which a branching/coupling device (abbreviated to BR/CPL in the figure) 3 branches a single transmission line extending from a station-side device (e.g., Subscriber Line Termination equipment, abbreviated to SLT in the figure) 1 into n transmission lines (hereinafter, abbreviated to XMSN LNs in the figures) 5-1, 5-2 to 5-n, each connected with subscriber devices (e.g., Optical Network Unit, abbreviated to ONUs) 2-1, 2-2 to 2-n (may be represented by 2), to perform bidirectional communication between a single device and a group of n devices.
To perform the bidirectional communication, upward signals (from ONU 2-1, 2-2, . . . , 2-n to SLT 1) are multiplexed with downward signals (vice versa) by the time division multiplexing (TDM) or wavelength division multiplexing (WDM).
As for the multiplexing of subscriber signals, the downward communication is performed by the TDM method in which a subscriber receives all the multiplexed signals transmitted from the SLT 1 and reads only the information within the time slot concerned. The upward communication is performed by the TDMA method in which the subscriber devices 2 sharing the station-side device 1 respectively control the signal transmission timing so that burst signals they transmit do not collide with each other.
Conventionally, wavelength division multiplexers (abbreviated to WDMs) 11-1, . . . , 11-n and supervisory units 6-1, . . . , 6-n were provided on the respective transmission lines 5-1, . . . , 5-n immediately after a branching/coupling device 3, viewed from the station-side device 1. The supervisory units 6-1, . . . , 6-n emit into the transmission lines 5-1, . . . , 5-n, a test-light pulse having a wavelength xcex2 which is different from wavelength xcex1 used for a user service (i.e., for carrying user data).
The respective supervisory units 6-1, . . . , 6-n receive the test-light pulse reflected at a fault point of the transmission lines 5-1, . . . , 2-n and locate the fault point based on the delay time (or propagation time) since the pulse is emitted until the reflected pulse returns to the supervisory units 6-1, . . . , 6-n and based on the power of the reflected-light pulse.
In the conventional method, the supervisory units 6-1, . . . , 6-n were provided in the respective transmission lines 5-1, . . . , 5-n, on the subscriber (ONU) side of the branching/coupling device 3 and adjacent to the station-side device 1 or in the station-side area. Thus, as many supervisory units and WDMs as the number of transmission lines are required, increasing the amount of hardware and therefore the cost of the supervising system. Moreover, since the distance of the transmission lines 5-1, . . . , 5-n between the branching/coupling device 3 and the respective ONU 2-1, . . . , 2-n increases, it is a problem that the probability of a transmission-line fault increases and the reliability of the optical communication system per se decreases.
A probable method to avoid the aforesaid problems by incorporating the supervisory units 6-1, . . . , 6-n with respective WDM 11-1, . . . , 11-n into one unit and placing the unit between the station-side device 1 and branching/coupling device 3, however, has following drawbacks. As is often the case with laying transmission lines to start communication services, if a fault occurs in two or more of transmission lines 5-1, . . . , 2-n, as many reflected-light pulses may return from the fault points. In this case, the supervisory unit can determine the distances to respective fault points but cannot determine a faulty transmission line. Further, if there exist plural fault points at the same distance from the supervisory unit, the reflected-light pulses from the fault points overlap with each other and the fault points may be recognized as one and it cannot be detected that faults occurred in plural transmission lines.
It is an object of the present invention to provide an ecconomical and reliable apparatus and method for locating a fault of a transmission line.
It is another object of the present invention to provide an apparatus and a method which can properly determine a faulty transmission line and precisely locate a fault point even if a fault occurs in plural transmission lines at the same time and even if a fault occurs in different lines at an equal distance from the supervisory unit.
It is still another object of the present invention to provide an apparatus and a method which can locate a fault of a transmission line, irrespective of whether or not the communication system is being brought into service.
It is a further object of the present invention to provide an apparatus and a method which can locate a fault of a transmission line without disturbing the communication and with a minimum load imposing on the line.
To achieve the above and other objects, the present invention provides an attenuators and a supervisory unit.
(1) In an apparatus for locating a fault of a transmission line in a communication system including a first device which transmits and receives a signal via a first transmission line, a plurality of second devices and a branching/coupling device which branches the first transmission line into a plurality of second transmission lines each connected with the respective second devices, wherein the first device and the plurality of second devices perform bidirectional communication, the attenuators cause individual attenuation to the plurality of second transmission lines. The supervisory unit inserts a test signal in the first transmission line and detects a reflected signal of the test signal. The supervisory unit determines the distance to a fault point based on the time since the test signal is inserted until the reflected signal is detected. The supervisory unit also compares for each of the second transmission lines, the attenuation caused by the attenuators with attenuation of the reflected signal and based on the comparing, determines a faulty one of the second transmission lines.
(2) In the aforesaid apparatus, the attenuators cause individual attenuation to the plurality of second transmission lines based on a specified attenuation condition. The supervisory unit specifies to the attenuators the attenuation condition including timing for causing attenuation to the second transmission lines, inserts a test signal in the first transmission line and detects a reflected signal of the test signal. The supervisory unit determines a distance to a fault point based on the time since the test signal is inserted until the reflected signal is detected. The supervisory unit also compares for each of the second transmission lines, the attenuation caused by the attenuators with attenuation of the reflected signal and based on the comparing, determines a faulty one of the second transmission lines.